1. Field of the Invention
The present invention relates to an image forming apparatus to form images according to an electrophotographic system such as a copying machine, printer, etc.
2. Description of the Related Art
Electrophotographic copying machines, printers, etc. are widely known as image forming apparatus. On electrophotographic copying machines, etc., after a photosensitive drum is uniformly charged by a main charger, an original document placed on a document table is exposed and by its reflected light is focused on the photosensitive drum, an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image on the photosensitive drum is developed by supplying toner particles from a developing device and a toner image is formed on the photosensitive drum. The formed toner image is transferred on a transfer paper that is charged by a transfer charger and a reproduced image is formed on this transfer paper.
On this kind of electrophotographic copying machines, copying speed has been increased in recent years. For the reasons of the increases in the diameter and peripheral speed of the photosensitive drum, etc. pursuant to the increase in the copying speed, the transfer efficiency of toner image electrostatically adhered on the photosensitive drum tends to become worse.
That is, when a toner image formed on the photosensitive drum is transferred on a transfer paper, if electrostatic adsorbing power of a toner to the photosensitive drum is too large, a toner image on the photosensitive drum is not sufficiently transferred on a transfer paper. As a result, defective images such as, for instance, a blurring of image on a transfer paper, insufficient image density, worse image minuteness, and transfer void are generated. These defects tend to increase pursuant to the increase in the copying speed.
In order to efficiently transfer a toner image electrostatically adhered to a photosensitive drum on a transfer paper, a charge removing means is often provided after developing an image and before transferring it. This pre-transfer charge removing means functions to weaken the electrostatic adhesive power between a toner and a photosensitive drum by removing the electric charge after developing and before transferring an image and efficiently transfer a toner on a transfer paper.
This pre-transfer charge removing means is broadly divided into two: a pre-transfer charge removing light source (PTL) to remove the electric charge on a photosensitive drum and a pre-transfer charger (PTC) to remove the electric charge on a photosensitive drum by the discharge from a wire, etc. and to improve the transfer efficiency of a toner itself.
The pre-transfer charge removing light source (PTL) is a means to weaken the electrostatic adhesive power of a toner to a photosensitive drum by removing the electric charge on a photosensitive drum by applying the light and a cold-cathode tube, LED, etc. are generally used. Further, with the increased speed of copying machines in recent years, it has become desirable to increase the transfer efficiency and for this purpose it is effective to increase the quantity of light of the pre-transfer charge removing light source. In many cases, a cold-cathode tube having a larger quantity of light than an LED is used for a pre-transfer charge removing light source.
The pre-transfer charger (PTC) is a means to weaken the electrostatic adhesive power between a photosensitive drum and a toner by discharging the photosensitive drum and charging the toner by a charger to facilitate the transfer of a toner image on a transfer paper. For this pre-transfer charger, a device to discharge DC charge of charged polarity and reverse polarity (that is the same polarity as a toner) of a photosensitive drum from a such very thin wire as platinum, tungsten oxide, etc. or a device to discharge AC charge after DC biased to the charged polarity and reverse polarity (that is, the same polarity of a toner) are used.
As described above, in case of electrophotographic copying machines, especially on copying machine with a photosensitive drum having increased speed for increasing a copying speed, a cold-cathode tube is often used as a pre-transfer charge removing light source to promote the toner image transfer efficiency in order to increase the transfer efficiency. However, when a cold-cathode tube is used as a pre-transfer charge removing light source, such problems as shown below are pointed out as characteristics of the cold-cathode tube.
That is, a large fluctuation of the quantity of light of a cold-cathode tube is pointed out. After a cold-cathode tube was continuously turned ON, that is, after the continuous copying by a copying machine, the cold-cathode tube was turned OFF and left for an hour and then turned ON, the quantity of light immediately after the cold-cathode tube is turned ON is low and gradually becomes clear.
Therefore, the quantity of light of a cold-cathode tube is set up so that the quantity of light when it is kept continuously turned ON prevents such defective images as transfer voids, etc. In this case, the quantity of light of the pre-transfer charge removing light becomes insufficient immediately after it is turned ON after left for an hour, the transfer efficiency may become worse and the transfer void can result. For this reason, after left for an hour, it is necessary to set the quantity of light of a cold-cathode tube immediately after it is turned ON at a sufficient quantity of light so as not to generate the transfer void. However, in this case the quantity of light of a cold-cathode tube when continuously kept ON becomes excessive and defects may be caused on an image such as image memory, etc. That is, when using a cold-cathode tube, the optimum width of quantity of light generating no image defects both immediately after and during when continuously kept ON is narrow (the margin of the quantity of light for transfer void and image memory is less).
Further, a cold-cathode tube has a characteristic that its quantity of light decreases under a low temperature environment, for instance, under environmental conditions of 5-10.degree. C., 20 RH %. Therefore, when a cold-cathode tube is used under a low temperature environment, the quantity of light becomes insufficient and transfer void and other image defects are generated.
On the other hand, the pre-transfer charger discharges a photosensitive drum by discharging the reverse polarity for the charged polarity of a photosensitive drum. As this is equivalent to the same polarity charge as a toner, a toner of the image portion is charged to increase the transfer efficiency. However, some quantity of weakly charged toner adhered on a non-image portion (the white ground), so-called fog toner on a photosensitive drum, is also charged. This weakly charged toner is conveyed to a cleaning portion without being transferred if there is no pre-transfer charger, however, as it is easily transferred if charged by a pre-transfer charger, the fog on a transfer paper becomes very large. Thus, use of a pre-transfer charger may be troublesome in that the fog level becomes worse.
Defective images, such as fog caused by a pre-transfer charger occur most frequently when a photosensitive drum nears the end of its service life (due to increase of the white ground potential) or under humid conditions (due to decrease in developer charged amount).